USE OF GSO FOR INELASTIC GAMMA-RAY SPECTROSCOPY MEASUREMENTS IN THE BOREHOLE

Gadolinium oxyorthosilicate (GSO), with its high density, high effective Z, fast decay time, and good performance at high temperature, is the scintillation detector of choice for a new generation of neutron-induced, inelastic gamma-ray spectroscopy tools for borehole measurements, specifically aimed at measuring the carbon and oxygen content from the region surrounding the borehole. The ratio of the carbon and oxygen gamma-ray yields are transformed into formation oil saturation. Because of the high energies of the carbon and oxygen gamma rays, 4.4 and 6.1 MeV, respectively, lower density and lower Z detectors cannot provide satisfactory results in small diameter tools. To get the maximum benefit from GSO, the data acquisition, choice of materials, and measurement geometry were optimized for small tool size and crystal characteristics. The large neutron cross section of the gadolinium in the scintillation crystal, its second decay constant, and the desired precision of the oil saturation measurement had important consequences in the tool design.